Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669515
Title: The transport and deposition of molybdenum in porphyry ore systems
Author: Spencer, Edward
ISNI:       0000 0004 5369 0682
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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Abstract:
Approximately 99% of Mo is obtained from porphyry deposits (e.g. Sillitoe, 2010), representing the strongest association of any metal with a single deposit type. Despite this, the controls on the formation of Mo-rich porphyries and the spatial-temporal decoupling of Mo from Cu in these systems remain poorly understood. By combining fieldwork, Re-Os molybdenite dating and a fluid inclusion study at one of the world's largest Cu-Mo porphyries (El Teniente, Chile), this project assesses the magmatic-hydrothermal evolution of the system to help constrain the processes controlling Mo enrichment. Results indicate that deposit-wide Mo grades of < 0.06 wt.% are principally controlled by the abundance of main mineralisation-type quartz-molybdenite veins, that develop outwards and upwards into Cu-rich vein types. Parental ore fluids had low salinities (~6.5 wt.% NaCleq) and were exsolved at depth following the emplacement of multiple intrusions that acted as short-lived (< 100,000 years) conduits for mineralising fluids between 6.3 and 4.6 Ma. The decoupled deposition of Mo and Cu surrounding each temporally separate intrusion is attributed to the sequential deposition of Mo then Cu in response to decreasing temperature followed by intermittent fluid boiling and increasing fluid pH in response to sericitic alteration of the mafic host-rocks. Mo grades > 0.06 wt.% correspond to the presence of late mineralisation-type veins and breccias produced by the exsolution of Mo-rich brines ┬▒aqueous fluids from a highly fractionated magma source. Mineralisation in this stage was short-lived and occurred in all parts of the deposit at ~4.6 Ma. The relatively Cu-poor nature of this stage is attributed to the prior preferential extraction of Cu from the underlying magma chamber in earlier mineralising events. In accordance with data from other Mo-rich porphyry systems, results indicate that Mo-Cu decoupling is controlled by both changes in magma chemistry as well as by the evolving P-T-X properties of hydrothermal fluids.
Supervisor: Wilkinson, Jamie; Berry, Andrew Sponsor: Imperial Janet Watson Scholarship
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.669515  DOI: Not available
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